The present invention relates to the texturing of magnetic data storage media, and more particularly to the texturing of dedicated transducing head contact zones (also called landing zones) of such media to reduce transducing head flying heights while also minimizing stiction.
Laser treated magnetic disks, particularly those textured over areas designed for contact with data transducing heads, are known to reduce friction and improve wear characteristics as compared to mechanically textured disks. Traditional laser texturing involves focusing a laser beam onto a disk substrate surface at multiple locations, forming at each location a depression surrounded by a raised rim as disclosed in U.S. Pat. No. 5,062,021 (Ranjan) and U.S. Pat. No. 5,108,781 (Ranjan). An alternative, as disclosed in International Publications No. WO 97/07931 and No. WO 97/43079, is to use a laser beam to form domes or nodules, rather than rims. In some cases, each of the domes is surrounded by a raised rim. The features can have either circular or elliptical profiles.
Collectively, the texturing features form a texture pattern or distribution throughout the head contact zone. A particularly preferred pattern is a spiral, formed by rotating the disk at a controlled angular speed while moving a laser radially with respect to the disk. The laser is pulsed to form the individual texturing features. The disk rotational speed and pulsing frequency together determine the circumferential pitch, i.e., the distance between adjacent texturing features in the spiral. Meanwhile, the radial speed of the laser controls the radial pitch or spacing between subsequent turns of the spiral. Frequently, the circumferential pitch and radial pitch are approximately the same, e.g., 20-30 microns. This spacing results in multiple texturing features cooperating to support a data transducing head at rest in the landing zone, given that the length and width dimensions of transducing head sliders typically are in the millimeter range.
The texturing features themselves can be made with a high degree of uniformity in size and shape, by maintaining a consistent laser power, focal spot size and pulse duration. As transducer glide heights (flying heights) continue to decrease, particularly below 1 microinch (25 nm), it becomes increasingly difficult for a texture to accommodate the glide height, and at the same time minimize stiction. The following table illustrates different measurements of three values, all in nm: an average height of multiple texturing features (Rp); the standard deviation of the height ("sgr"); and the measured glide avalanche. Glide avalanche occurs when a measured signal output exceeds a certain threshold, indicating that the transducing head is flying xe2x80x9ctoo closexe2x80x9d to the surface.
From the table, it is seen that to avoid undue risk of collisions of the transducing head with the texturing features, while at the same time maintaining a flying height of about 25 nm, the average height of the features should be less than about 20 nm.
At the same time, the need to minimize friction and stiction imposes limits on the minimum heights of the texturing features. Typically, a liquid lubricant is applied to the surfaces of magnetic data storage disks, to improve wear characteristics and reduce dynamic friction. However, the liquid lubricant has the undesirable effect of contributing to stiction, the tendency of a data transducing head, once at rest against a magnetic disk, to adhere to the disk. This provides at least momentary resistance when the disk begins to rotate, potentially damaging both the disk and the transducing head, and risking loss of data.
A primary cause of stiction is the tendency of the liquid lubricant, through capillary action, to flow about and surround the texturing features in contact with an at-rest transducing head, even flowing to the head itself as indicated in FIG. 1, where h indicates the height of a data transducing head when at rest upon the texturing features, two ofwhich are shown The texturing features, nodules or bumps, are flattened slightly by the head over an area of contact with a radius r. By comparison, rm is the radius of the liquid lubricant meniscus surrounding each texturing feature, with each feature having a radius R at its base. The value d represents the thickness of the liquid lubricant film over surface areas away from the nodules. Thus, a meniscus of the liquid lubricant surrounds each texturing feature, occupying the full height between the transducing head and disk surface, clinging to the transducing head to provide momentary resistance to disk acceleration.
As seen in the chart of FIG. 2, the stiction effect increases dramatically as the height of texturing features decreases below 12 nm.
In view of the above, one option for achieving a 1 microinch glide height while minimizing stiction appears to be maintaining texturing feature heights within the range of 13-19 nm. However, given the lack of absolute precision in laser beam generation and optical components that focus and otherwise shape the laser beam, variance in substrate materials and parameters, and variance in the slider bodies that aerodynamically determine transducing head flying heights, the required degree of control is not practical. Further, when the heights of texturing features are reduced, their diameters are reduced as well, and it may be desirable to maintain larger diameter nodules or other features to enhance structural stability.
Therefore, it is an object of the present invention to provide a substrate for a data storage medium having a landing zone textured to accommodate glide heights less than 25 nm, while simultaneously minimizing stiction.
Another object is to provide a landing zone surface texture utilizing larger texturing features with greater heights, in combination with recessed regions surrounding the texturing features to accommodate liquid lubricant and thereby counteract the tendency of capillary flow toward a transducing head at rest on the landing zone.
A further object is to provide an improved process for texturing substrates and for fabricating magnetic data recording media to exhibit improved resistance to stiction despite lower transducing head flying heights.
Yet another object is to provide a data storage medium having improved wear characteristics and the tendency to afford increased longevity to data transducing heads used in conjunction with the medium.
To achieve these and other objects, there is provided a substrate for a data storage medium of the type including a data zone for storing data and a landing zone textured for contact with a data transducing head maintained in spaced apart relation to the data storage medium during use. The substrate includes a substrate body having a substantially planar substrate surface at least over a landing zone thereof. A recessed region is disposed within the landing zone, and has a substantially planar recessed surface spaced apart inwardly from the substrate surface by a predetermined distance substantially uniform throughout the landing zone. Multiple texturing features are formed in the recessed region and are projected outwardly from the recessed surface by a projection distance which exceeds the predetermined distance. Consequently, the texturing features project outwardly beyond the substrate surface of the substrate body.
This texture can be conveniently thought of as a xe2x80x9cdual baselinexe2x80x9d texture, in that it provides two separate baselines: one with regard to transducer flying height, and the other with regard to meniscus formation. In particular, the meniscus formation or xe2x80x9cstictionxe2x80x9d baseline is the recessed surface of the recessed region. To reach a transducing head at rest in the landing zone, a liquid lubricant would be required to traverse the height of each texturing feature, beginning at the base of the texturing feature, i.e., the recessed surface.
In contrast, the baseline for glide height or flying height is the geometric mean of the substrate surface and the recessed surface. By forming the recessed region as a small fraction of the landing zone, e.g., one-third of the surface area or less, the geometric mean is substantially nearer to the substrate surface. Accordingly, the texturing feature heights, as they relate to transducing head flying height, are effectively reduced by a fraction (preferably at least one-half, more preferably at least two-thirds) of the xe2x80x9cpredetermined distancexe2x80x9d between the substrate surface and the recessed surface.
Thus, the benefits of reduced transducing head flying height and reduced stiction are simultaneously achieved, and in degrees that can vary according to design considerations. For example, by setting a projection distance, or texturing feature height above the recessed surface, at slightly more than 12 nm, stiction can be kept acceptably low while extremely low, sub-microinch flying heights are achieved. Alternatively, the texturing features can be formed to project beyond the substrate surface by slightly less than 20 nm, to maintain an acceptably low flying height while considerably diminishing the chance for stiction due to meniscus formation.
Preferably, the xe2x80x9cpredetermined distancexe2x80x9d or separation between the substrate surface and recessed surface is at least about 5 nm, and more preferably is in the range of about 5 to about 10 nm.
To further ensure against meniscus formation, there should be a clearance between each of the texturing features and the nearest edge of the recessed region, as measured in directions parallel to the substrate and recessed surfaces. The clearance should be at least 3 nm, and more preferably is about 5 nm or more.
Another aspect of the present invention is a process for selectively texturing a recording medium substrate, including the following steps:
a. providing a substrate body having a substantially planar substrate surface;
b. applying a material layer over the substrate surface at a substantially uniform thickness, whereby an outer surface of the material layer is substantially planar and parallel to the substrate surface;
c. selectively removing portions of the material from the material layer within a predetermined zone thereof, to provide a selected region with a recessed surface disposed inwardly of the outer surface of the material layer; and
d. forming multiple texturing features throughout the selected region, each texturing feature projecting outwardly away from the recessed surface and beyond the outer surface of the material layer.
A preferred material is carbon, applied by vacuum deposition to a thickness of about 5-10 nm. Then, a laser (e.g., a CO2 laser) is used to remove the carbon at selected locations or spots. Preferably, the laser ablation removes the carbon throughout the thickness of the carbon layer, thus leaving the substrate body, e.g., a glass ceramic, exposed. As a result, the thickness of the carbon layer provides the uniform separation distance between the substrate surface and the outer surface. Then, another laser (e.g., a YAG laser) is directed onto the exposed substrate surface areas to form the texturing features, preferably in a one-to-one correspondence to the areas of carbon removal, and preferably with the texturing feature centered within its associated carbon-depleted spot or location.
At this stage, texturing of the landing zone is complete. As an option, that portion of the carbon layer spanning the data zone of the disk is removed.
Next, post-texturing steps of disk fabrication are completed. These include the application of several further layers to the textured substrate including the data zone and landing zone. These layers include a chromium underlayer, a thin film magnetic recording layer, and a protective cover layer, typically carbon. These subsequent layers are applied by vacuum deposition and in substantially uniform thicknesses, such that the outer surface of the protective carbon layer replicates the topography of the textured substrate.
Thus, in accordance with the present invention, the landing zone of a data storage medium substrate is provided with separate, spaced apart baselines with respect to transducer flying height and stiction. The separate baselines are provided by forming a recessed region within the landing zone, and forming texturing features only within the recessed region. Consequently, the features can have actual heights, with respect to the stiction baseline, sufficient to minimize stiction due to meniscus formation, yet also have effective heights, with respect to the non-recessed substrate surface, sufficiently small to accommodate sub-microinch transducer flying heights.